In the past such downwardly directed luminaires, hereinafter referred to as downlights, have provided a generally right circular cone of light which, because of its particular shape, is able to satisfy particular spacing requirements. That is, for a particular downlight, the distance between adjacent downlights relative to the height above the work plan was relatively fixed. One such downlight, marketed by the assignee of the present application, is a luminaire marketed by Holophane under the trade designation LOBAY PRISMPACK I. This downlight has a reflector having generally a shape of a surface of revolution. The inner reflective surface is provided with a white baked enamel finish. Below the lamp, positioned along the axis of the reflector is a refractive lens which, because of its particular characteristics modifies the light (emanating directly from the lamp and being reflected from the white reflective surface of the reflector) to control light therefrom and redirect it in a generally downwardly direction. This reduces glare producing light, i.e., light which would emanate from the light/reflector combination at relatively high angles to nadir. Preferably this lens is an acrylic REFRACTIVE GRID lens (registered trademark of the Johns-Manville Corporation). This lens is injection molded and consists of a precisely formed array of hemispherical refractive elements on both the upper and lower surfaces thereof. These elements freely transmit light downwardly while intercepting potential glaring rays and redirecting them into the conical beam. U.S. Pat. No. 3,763,369 discloses the details of this refractive grid lens and is hereby incorporated by reference.
The overall combination provided by the above disclosed downlight, while quite effective in providing reasonably uniform illumination within the conical beam, provides a spacing to mounting height ratio of 1.5. That is, in order to provide uniform illumination to the work plane a series of such downlights must be spaced about 11/2 times their mounting height from one another. Unfortunately, industrial buildings are now being constructed with relatively low ceiling heights. Also, such new industrial buildings or industrial buildings being refurbished tend to have "finished ceilings" further lowering the maximum mounting height for downlights. With the advent of H.I.D. (high intensity discharge) light sources, higher lighting levels provided by these sources require that the units be spread out further in order to avoid providing higher than required illumination at the work plane. Thirdly, despite the ability of such downlights to provide higher illumination levels, there is a trend to design for even lower levels than in the past, with an eye to the energy shortage and the higher cost of providing electrical energy to maintain such lighting levels.